In liquid crystal display elements, such as liquid crystal cells which operate by the action of an electric field, a liquid crystal oriented film (insulating film) is formed so as to cover a surface of an electrode formed on a substrate, such as a glass plate, in order to prevent direct contact between the electrode and liquid crystals, which causes deterioration of the liquid crystals. The liquid crystal oriented film conventionally employed is an inclined deposited film of silicon oxide. However, the silicon oxide deposited film is not a good liquid crystal oriented film because it shows considerably different orientation characteristics depending on the type of liquid crystals used. Hence, it has been proposed to use a rubbed film of an organic high polymer as a liquid crystal oriented film. While many of the rubbed high polymer films have a disadvantage that the rubbing effect is reduced by heating for assembling a liquid crystal cell, polyimide films are free from such a disadvantage. Therefore, aromatic polyimide films having satisfactory orientation controllability are widely employed.
Since an aromatic polyimide is generally insoluble and infusible, it is usually coated on a substrate in the form of a polyamide acid, i.e., a precursor of a polyimide, dissolved in an organic polar solvent, such as N-methyl-2-pyrrolidone, and the precursor is then imidized by heating at high temperatures, accompanied by dehydrocyclization, to cure the same. The polyimide film for use as liquid crystal oriented film has also been formed in the same manner.
However, the thus formed polyimide films are colored brown due to the severe thermal history up to the film formation. Such coloration darkens the visual field to reduce contrast, thereby impairing functions of display elements. Thus, the polyimide films do not respond to the demand for high quality liquid crystal display elements. For example, polyimide films prepared from pyromellitic acid dianhydride and 4,4'-diaminodiphenylether as starting materials are fairly excellent in homogeneity of orientation and durability, but they do not satisfy requirements for high quality liquid crystal display elements due to their brown color which arises from high temperature heating at dehydrocyclization (imidation). In addition, aromatic polyimide films are generally inferior in adhesion to a substrate to cause penetration of water into the interface between the substrate and the polyimide film, resulting in decreasing reliability of the liquid crystal display elements.
Other than the above-described polyamide acid solution for forming liquid crystal oriented film, polyamide acid solutions used for production of substrates of solar cells, coating the surface of lenses, and the like have the requirements of adhesion to a substrate and transparency of a film formed, but satisfactory polyamide acid solutions are not yet obtained.
Thus, it is extremely difficult for the conventional prior art to form a colorless transparent polyimide film having excellent adhesion to a substrate, which is particularly useful as a liquid crystal oriented film, and the improvement thereof is strongly demanded